The oligomerisation of olefins, primarily α-olefins, with chromium catalysts has been extensively studied. More specifically, a number of chromium catalysts have been developed and used to trimerise olefins. In this regard, the trimerisation of ethylene to 1-hexene is significant since, in addition to its use as a specific chemical, 1-hexene is extensively used in polymerization processes either as a monomer or co-monomer. Furthermore, the trimeric products derived from longer chain olefins could be well utilized as synthetic lubricants (e.g. polyalphaolefins/PAOs), as well as various other applications such as components of drilling muds, and as feedstock to prepare detergents and plasticizers.
Prior art chromium based ethylene trimerisation processes include:    a) A process in which olefins are trimerised by passing the olefin in contact with a catalyst comprising the reaction product of a chromium compound, an organoaluminium compound hydrolyzed with a specific amount of water and a donor ligand selected from hydrocarbyl isonitriles, amines and ethers (U.S. Pat. No. 4,668,838);    b) A process to trimerise ethylene to 1-hexene comprising contacting ethylene with a stabilized catalyst system comprising a chromium source, a pyrrole-containing compound, a metal alkyl and an aromatic compound (European Patent No. 0 668 105);    c) A process for preparing α-olefin oligomers, which comprises carrying out oligomerisation of an α-olefin in a solvent by reacting said α-olefin with a chromium-based catalyst system comprising a combination of at least a chromium compound, an amine or metal amide, and an alkylaluminium compound, in a contacting mode that the chromium compound and the alkylaluminium compound are not previously contacted with each other (U.S. Pat. No. 5,750,817);    d) A process for oligomerising ethylene to produce 1-butene and/or 1-hexene wherein catalytic composition is obtained by mixing at least one chromium compound with at least one aryloxy aluminium compound with general formula RnAl(R′O)3−n where R is a linear or branched hydrocarbyl radical containing 1 to 30 carbon atoms, R′O is an aryloxy radical containing 6 to 80 carbon atoms and n is a whole which can take the values 0, 1 or 2, and with at least one other hydrocarbyl aluminium compound selected from tris(hydocarbyl)aluminium compound or chlorinated or brominated hydrocarbyl aluminium compounds (U.S. Pat. No. 6,031,145); and    e) A process for the trimerisation of ethylene, said process comprising reacting ethylene, using a catalyst comprising an aluminoxane and a polydentate phosphine, arsenic and/or antimony coordination complex of a chromium salt, such that 1-hexene is formed (U.S. Pat. No. 5,811,618).